In recent years, increasing social demand has been emerging to make efficient use of frequency resources, along with increasing widespread of mobile stations such as PHS (personal handyphone system) handsets and mobile telephones. One of the communication methods that responds to this demand is a spatial multiplexing method.
The spatial multiplexing method is a communication method for multiplexing transmission and reception signals to and from a plurality of mobile stations and achieving simultaneous communication between the mobile stations on a single frequency, by forming a different directivity pattern for each mobile station with the use of an adaptive array apparatus.
An adaptive array apparatus is equipped with a plurality of antennas, and adjusts the amplitude and phase of the transmission and reception signals of each antenna, to form a directivity pattern (an array antenna pattern) of the antennas as a whole.
A wireless base station that wirelessly connects a plurality of mobile stations by spatial multiplexing is constructed to use such an adaptive array apparatus. To separate a reception signal intended for each mobile station from the multiplexed reception waves transmitted from the plurality of mobile stations, the wireless base station calculates, for each mobile station, a weight coefficient (also referred to as a “weight vector”) that is used to adjust the amplitude and phase of reception waves (reception signals) of each antenna. A weight vector can be calculated by a DSP (Digital Signal Processor) in the following way. In the equation below, the DSP adjusts values of “W1 (t−1)” to “W4 (t−1)” so as to minimize an error range “e(t)”. The DSP then sets the adjusted values of “W1 (t−1)” to “W4 (t−1)” as weight vectors “W1 (t)” to “W4 (t)” for a symbol at the timing “t”.
Equatione(t)=d(t)−(W1(t−1)★X1(t)+W2(t−1)★X2(t)+W3(t−1)★X3(t) +W4(t−1)★X4(t))
Here, the legend “t” represents a timing in symbol units, the legend “d(t)” represents symbol data in a known reference signal (or in a training signal), the legends “X1 (t)” to “X4 (t)” represent reception signals of four antennas, and the legends “W1 (t−1)” to “W4 (t−1)” represent initial values for weight vectors of these four antennas. It should be noted here that the initial values may be freely chosen, but weight vectors of these four antennas calculated for the preceding symbol or in the preceding reception timeslot are generally used as the initial values.
In short, weight vectors are calculated so as to minimize a difference between (a) a sum of values each obtained by multiplying the reception wave (reception signal) of each of the four antennas by its weight vector, and (b) the reference signal. The reference signal includes bits (or symbol data) of a known bit sequence (or a symbol sequence) contained in a control signal on a control channel or a communication signal on a communication channel. For PHS, for example, fixed bit sequences, such as PR (preamble) and UW (Unique Word), contained in reception signals are used as the reference signal.
As described above, the wireless base station separates a reception signal intended for each mobile station from multiplexed reception waves, by calculating, for each of a plurality of mobile stations that are to be spatially multiplexed, a weight vector of each antenna and weighting the multiplexed reception waves transmitted from the plurality of mobile stations. At the time of transmission, the wireless base station forms a directivity pattern by weighting transmission signals using the weight vectors calculated at the time of reception. It should be noted here that spatial multiplexing is also referred to as PDMA (Path Division Multiple Access), and is described in detail in “Mobile Communication using PDMA” in Shingaku Giho (Communication Studies) RCS 93–84 (1994-01), pp37–44.
The calculation of weight vectors and separation of signals are easy if a value of the above reference signal varies depending on each of the mobile stations that are to be connected wirelessly by spatial multiplexing. In the case of PHS, however, the fixed bit sequences such as the above-mentioned PR and UW, used as the reference signal, are common to every mobile station. In some cases, therefore, correct weight vectors cannot be calculated, and accordingly a signal cannot be separated accurately. To be more specific, in the multiplexed reception signals from the plurality of mobile stations, if the center frequencies of signals intended for all the mobile stations completely match, and timings of symbols-the minimum unit of transmission and reception data-for all the mobile stations completely match, correct weight vectors cannot be calculated, and therefore a desired signal cannot be separated.
However, each mobile station actually generates an internal timing clock and a carrier wave frequency signal on its own, which inevitably creates an error range of several ppm. Accordingly, it is extremely unusual that the symbol timings for all the mobile stations completely match and at the same time the center frequencies of carrier waves intended for all the mobile stations completely match.
By taking advantage of this fact, it is considered possible for the wireless base station to calculate correct weight vectors, by detecting a deviation of the symbol timing and a deviation of the carrier wave frequency for each mobile station, and reflecting the detected deviations in reception waves of each antenna.
However, the above conventional technique has the following problem. At the time of transfer from a control channel to a communication channel, i.e., when new wireless connection to a mobile station is performed by spatial multiplexing, a deviation of the symbol timing and a deviation of the carrier wave frequency of the mobile station are unknown to the wireless base station. Accordingly, the wireless base station has a low chance of calculating correct weight vectors for the mobile station. This may even cause a failure in establishing a communication channel to initiate spatial multiplexing of the mobile station. If such a failure occurs, the wireless base station has to retry establishing another communication channel.
For example, mobile stations such as PHS handsets and mobile telephones use a control channel for a standby mode to receive a call and a communication channel for communication. Therefore, immediately after the transfer from a control channel to a communication channel that is subjected to spatial multiplexing at the time of call-in or call-out, a deviation of the symbol timing and a deviation of the carrier wave frequency are unknown. This is no problem if the wireless base station could detect these deviations immediately after the transfer to the communication channel, but actually, the wireless base station is not able to detect these deviations until separating a signal intended for the newly connected mobile station using correct weight vectors. This means that the wireless base station cannot utilize a deviation of the symbol timing and a deviation of the carrier wave frequency immediately after the transfer from a control channel to a communication channel, and accordingly, cannot calculate weight vectors with high accuracy.
In view of the above problem, the object of the present invention is to provide a wireless base station that improves accuracy of separating a signal intended for a mobile station when new wireless connection to the mobile station is performed by forming an antenna directivity, and that ensures establishment of a communication channel.